XII PREFACE. 
The investigations of these three engineers were conducted inde- 
pendently, and their reports have been prepared without collabora- 
tion. Under these circumstances there are necessarily some differ- 
ences of opinion as to matters of detail, but as to the more important 
features the writers are in hearty accord. About four hundred illus- 
trations were submitted with the original reports; many of these do 
not appear with the printed reports because their use would have 
involved duplication, but wherever a view given by one author was 
rejected because of its similarity to a view by another author show- 
ing the same engineering features, a reference to the accepted view 
has been inserted. The legend appended to each illustration indi- 
cates whether the original view was actually taken by the author or 
was procured from another source. 
Persons interested in this subject are advised not only to read each 
of these papers, but also to consult a number of other important 
papers on this subject which have appeared in the different technical 
journals and the proceedings of technical societies during the last 
several months. (See list at end of bulletin, pp. 159-161.) 
